Wabe and Thundering1 have stated all this quite correctly.  I print on an Epson 9800 giclee wide format printer (to 44' wide).  I upsample my rendered images with Lanczos or B-spline filters x2 to obtain a print resolution of about 240 DPI or higher (minimum I use is 200 DPI).

File size is determined (all things being equal) by horizontal and vertical pixel count.  Pixel resolution numbers are related to file size, DPI defines pixel density.  The larger the pixel count, the more available information in the image. The greater the pixel density (DPI), the finer the information is displayed per square inch (or cm -- if using metric).